Polymers stabilized by protective colloids are used in particular in the form of their aqueous dispersions or water-redispersible polymer powders in diverse applications, as coating compositions or adhesives, for example, for a very wide variety of substrates: for example, as cementitious tile adhesives. The protective colloids used are generally polyvinyl alcohols. The use of polyvinyl alcohol is worthwhile because, in comparison to systems stabilized by low molecular mass compounds, such as emulsifiers, polyvinyl alcohol makes its own contribution to strength (e.g., tensile adhesion values in the tile adhesive). Monomers used to prepare redispersible powders have to date been preferably vinyl esters and ethylene. When the vinyl ester/ethylene combination is used, the establishment of stability with respect to saponification necessitates a high ethylene content, which reduces the Tg. This may have adverse effects on the tensile bond strength of the tile adhesives, especially after storage under hot conditions. What is desirable, however, is a high and constant level of the tensile bond strengths in the tile adhesive after storage under any conditions (dry, wet, heat and freeze/thaw).
Especially when polymers are used in the form of their redispersion powders to improve the properties of mortar, a major field of use for redispersion powders, the formulations are required to remain stable for a certain time and must not suffer any substantial alteration in their processing consistency (viscosity stability or cement stability); indeed, a user cannot be expected to have to stir up a new mixture within a short time span. In the concrete and mortar industry, furthermore, an important part is played by the mechanical properties, such as the compressive strength, the porosity and thus the air pore content. If there are too many air pores, the compressive strength falls sharply; if there are too few, or no, air pores in the mortar or concrete, the building material will lack sufficient freeze/thaw stability.
DE-A 21 48 456 (GB-A 14 07 827) discloses building adhesives based on copolymers containing hydroxyl-containing silicon compounds in copolymerized form. EP-B 35332 describes tile adhesives based on styrene-acrylate copolymers, containing not only hydroxyl-containing silicon compounds but also relatively large amounts—up to 10% by weight—of copolymerized (meth)acrylic acid units. In EP-A 640630, it is recommended, for the purpose of preparing tile adhesives, to use silicon-modified polymer dispersions containing up to 4% by weight of ethylenically unsaturated carboxamides in copolymerized form.
It is an object of the invention to provide stable, low-viscosity, protective colloid stabilized dispersion powders based on vinyl ester polymers which, when used in cementitious applications, possess a completely satisfactory viscosity stability or cement stability and which do not hinder the setting of the cement. A further object is to provide redispersible powders which have better and more consistent tensile bond strengths after storage under any conditions.